ChapterChapter 5454 Ecosystems PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Overview: Ecosystems, Energy, and Matter • An ecosystem consists of all the organisms living in a community, as well as the abiotic factors with which they interact • Ecosystems range from a microcosm, such as an aquarium, to a large area such as a lake or forest • Regardless of an ecosystem’s size, its dynamics involve two main processes: energy flow and chemical cycling • Energy flows through ecosystems while matter cycles within them Concept 54.1: Ecosystem ecology emphasizes energy flow and chemical cycling • Ecologists view ecosystems as transformers of energy and processors of matter Ecosystems and Physical Laws • Laws of physics and chemistry apply to ecosystems, particularly energy flow • Energy is conserved but degraded to heat during ecosystem processes Trophic Relationships • Energy and nutrients pass from primary producers (autotrophs) to primary consumers (herbivores) and then to secondary consumers (carnivores) • Energy flows through an ecosystem, entering as light and exiting as heat • Nutrients cycle within an ecosystem LE 54-2 Tertiary consumers Microorganisms and other detritivores Secondary consumers Detritus Primary consumers Primary producers Heat Key Chemical cycling Sun Energy flow Decomposition • Decomposition connects all trophic levels • Detritivores, mainly bacteria and fungi, recycle essential chemical elements by decomposing organic material and returning elements to inorganic reservoirs Concept 54.2: Physical and chemical factors limit primary production in ecosystems • Primary production in an ecosystem is the amount of light energy converted to chemical energy by autotrophs during a given time period Ecosystem Energy Budgets • The extent of photosynthetic production sets the spending limit for an ecosystem’s energy budget The Global Energy Budget • The amount of solar radiation reaching the Earth’s surface limits photosynthetic output of ecosystems • Only a small fraction of solar energy actually strikes photosynthetic organisms Gross and Net Primary Production • Total primary production is known as the ecosystem’s gross primary production (GPP) • Net primary production (NPP) is GPP minus energy used by primary producers for respiration • Only NPP is available to consumers • Ecosystems vary greatly in net primary production and contribution to the total NPP on Earth LE 54-4 Open ocean 65.0 125 24.4 Continental shelf 5.2 360 5.6 Estuary 0.3 1,500 1.2 Algal beds and reefs 0.1 2,500 0.9 Upwelling zones 0.1 500 0.1 Extreme desert, rock, sand, ice 4.7 3.0 0.04 Desert and semidesert scrub 3.5 90 0.9 Tropical rain forest 3.3 2,200 22 Savanna 2.9 900 7.9 Cultivated land 2.7 600 9.1 Boreal forest (taiga) 2.4 800 9.6 Temperate grassland 1.8 600 5.4 Woodland and shrubland 1.7 700 3.5 Tundra 1.6 140 0.6 Tropical seasonal forest 1.5 1,600 7.1 Temperate deciduous forest 1.3 1,200 4.9 Temperate evergreen forest 1.0 1,300 3.8 Swamp and marsh 0.4 2,000 2.3 Lake and stream 0.4 250 0.3 0 10 20 30 40 50 60 0 500 1,0001,500 2,000 2,500 0 5 10 15 20 25 Key Percentage of Earth’s Average net primary Percentage of Earth’s net surface area production (g/m2/yr) primary production Marine Terrestrial Freshwater (on continents) • Overall, terrestrial ecosystems contribute about two-thirds of global NPP • Marine ecosystems contribute about one-third LE 54-5 North Pole 60°N 30°N Equator 30°S 60°S South Pole 180° 120°W 60°W 0° 60°E 120°E 180° Primary Production in Marine and Freshwater Ecosystems • In marine and freshwater ecosystems, both light and nutrients control primary production Light Limitation • Depth of light penetration affects primary production in the photic zone of an ocean or lake Nutrient Limitation • More than light, nutrients limit primary production in geographic regions of the ocean and in lakes • A limiting nutrient is the element that must be added for production to increase in an area • Nitrogen and phosphorous are typically the nutrients that most often limit marine production • Nutrient enrichment experiments confirmed that nitrogen was limiting phytoplankton growth in an area of the ocean LE 54-6 30 21 and g Isl Shinnecock Lon 19 15 Bay 5 ay 4 h B 11 Moriches Bay out t S rea 2 G Atlantic Ocean Coast of Long Island, New York 8 Phytoplankton 8 7 Inorganic 7 6 phosphorus 6 5 5 4 4 3 3 (µm atoms/L) Phytoplankton 2 2 (millions of cells/mL) (millions 1 1 Inorganic phosphorus Inorganic 0 0 2 4 5 113015 19 21 Station number Great Moriches Shinnecock South Bay Bay Bay Phytoplankton biomass and phosphorus concentration 30 Ammonium enriched Phosphate enriched Unenriched control 24 18 12 Phytoplankton 6 (millions of cells per mL) of cells (millions 0 Starting 2 4 5 11 30 15 19 21 algal Station number density Phytoplankton response to nutrient enrichment • Experiments in another ocean region showed that iron limited primary production • The addition of large amounts of nutrients to lakes has a wide range of ecological impacts • In some areas, sewage runoff has caused eutrophication of lakes, which can lead to loss of most fish species Primary Production in Terrestrial and Wetland Ecosystems • In terrestrial and wetland ecosystems, climatic factors such as temperature and moisture affect primary production on a large scale • Actual evapotranspiration can represent the contrast between wet and dry climates • Actual evapotranspiration is the water annually transpired by plants and evaporated from a landscape • It is related to net primary production LE 54-8 3,000 Tropical forest /yr) 2 2,000 Temperate forest 1,000 Mountain coniferous forest Desert Temperate grassland shrubland Net primary production (g/m Net primary Arctic tundra 0 0 500 1,000 1,500 Actual evapotranspiration (mm/yr) • On a more local scale, a soil nutrient is often the limiting factor in primary production LE 54-9 300 N + P 250 ) 2 200 150 N only (g dry wt/m 100 Control Live, above-ground biomass Live, above-ground 50 P only 0 0 June July August 1980 Concept 54.3: Energy transfer between trophic levels is usually less than 20% efficient • Secondary production of an ecosystem is the amount of chemical energy in food converted to new biomass during a given period of time Production Efficiency • When a caterpillar feeds on a leaf, only about one- sixth of the leaf’s energy is used for secondary production • An organism’s production efficiency is the fraction of energy stored in food that is not used for respiration LE 54-10 Plant material eaten by caterpillar 200 J 67 J Cellular 100 J respiration Feces 33 J Growth (new biomass) Trophic Efficiency and Ecological Pyramids • Trophic efficiency is the percentage of production transferred from one trophic level to the next • It usually ranges from 5% to 20% Pyramids of Production • A pyramid of net production represents the loss of energy with each transfer in a food chain LE 54-11 Tertiary 10 J consumers Secondary 100 J consumers Primary 1,000 J consumers Primary producers 10,000 J 1,000,000 J of sunlight Pyramids of Biomass • In a biomass pyramid, each tier represents the dry weight of all organisms in one trophic level • Most biomass pyramids show a sharp decrease at successively higher trophic levels LE 54-12a Trophic level Dry weight (g/m2) Tertiary consumers 1.5 Secondary consumers 11 Primary consumers 37 Primary producers 809 Most biomass pyramids show a sharp decrease in biomass at successively higher trophic levels, as illustrated by data from a bog at Silver Springs, Florida. • Certain aquatic ecosystems have inverted biomass pyramids: Primary consumers outweigh the producers LE 54-12b Trophic level Dry weight (g/m2) Primary consumers (zooplankton) 21 Primary producers (phytoplankton) 4 In some aquatic ecosystems, such as the English Channel, a small standing crop of primary producers (phytoplankton) supports a larger standing crop of primary consumers (zooplankton). Pyramids of Numbers • A pyramid of numbers represents the number of individual organisms in each trophic level LE 54-13 Trophic level Number of individual organisms Tertiary consumers 3 Secondary consumers 354,904 Primary consumers 708,624 Primary producers 5,842,424 • Dynamics of energy flow in ecosystems have important implications for the human population • Eating meat is a relatively inefficient way of tapping photosynthetic production • Worldwide agriculture could feed many more people if humans ate only plant material LE 54-14 Trophic level Secondary consumers Primary consumers Primary producers The Green World Hypothesis • Most terrestrial ecosystems have large standing crops despite the large numbers of herbivores • The green world hypothesis proposes several factors that keep herbivores in check: – Plant defenses – Limited availability of essential nutrients – Abiotic factors – Intraspecific competition – Interspecific interactions Concept 54.4: Biological and geochemical processes move nutrients between organic and inorganic parts of the ecosystem • Life depends on recycling chemical elements • Nutrient circuits in ecosystems involve biotic and abiotic components and are often called biogeochemical cycles A General Model of Chemical Cycling • Gaseous carbon, oxygen, sulfur, and nitrogen occur in the atmosphere and cycle globally • Less mobile elements such as phosphorus, potassium, and calcium cycle on a more local level • A model of nutrient cycling includes main reservoirs of elements and processes that transfer elements between reservoirs • All elements cycle between organic and inorganic reservoirs LE 54-16 Reservoir a Reservoir b Organic Organic materials materials available unavailable as nutrients as nutrients Fossilization Living organisms, Coal, oil, detritus peat Respiration, Assimilation, decomposition, photosynthesis excretion Burning of fossil fuels Reservoir c Reservoir d Inorganic Inorganic materials materials available Weathering, unavailable as nutrients erosion as nutrients Atmosphere, Minerals soil, water Formation of in rocks sedimentary rock Biogeochemical Cycles • In studying cycling of water, carbon, nitrogen, and phosphorus, ecologists focus on four factors: 1.